code: def test_composite_mixin_len():
      b_1 = Bar()
      b_2 = Line()
      c = CustomCompositeChart()
      c.add(b_1, "bar")
      c.add(b_2, "line")
      assert len(c) == 2
  
file_path: pyecharts/test/test_mixins.py
imports: from unittest.mock import patch
  from nose.tools import assert_equal
  from pyecharts import options
  from pyecharts.charts import Bar, Line
  from pyecharts.charts.mixins import CompositeMixin
label_desc: Use image node_id 1 for calling a global function with example usage: test_composite_mixin_len() without return types